Tire state monitoring systems have conventionally been known which monitor a tire state quantity (e.g., air pressure, temperature, or the like) of pneumatic tires. Such a tire state monitoring system is provided with a tire state acquisition device which is disposed in a cavity portion of a pneumatic tire and acquires a tire state quantity (e.g., air pressure or temperature) and a monitoring device which receives radio waves from the tire state acquisition device to monitor the tire state quantity.
Additionally, collision prevention systems have conventionally been known which prevent a following vehicle from colliding with a host vehicle. Such a conventional collision prevention system issues an alert from the host vehicle to the following vehicle when a distance between the host vehicle and the following vehicle is small, the following vehicle is rapidly approaching the host vehicle, or the like, in order to catch the attention of the driver of the following vehicle. This configuration prompts the following vehicle to take appropriate measures such as ensuring a sufficient distance with respect to the host vehicle, which can prevent the following vehicle from colliding with the host vehicle. The techniques disclosed in Japanese Unexamined Patent Application Publication Nos. S60-024479A, H01-321600A and H08-193831A are known, conventional collision prevention systems.